Mapping variations in the human genome across populations is essential if scientists are ever going to understand its three billion letters and cure diseases, the world congress of genetics has heard in Melbourne.

Dr Francis Collins, director of the U.S. National Human Genome Research Institute, told the 2,600 delegates of the 19th Genetics Congress earlier today that compiling a 'haplotype map' of the human species would give clues to where the genes that help cause genetic ailments are hidden.

The Human Genome Project, which Collins has headed for more than a decade, finalised the sequence of three billion bases that make up humans in April 2002. Bases are the 'rungs' on the double helix spiral ladder of DNA, and there are four of them: A, G, C, T. Each gene is made up of a combination of these four-letter codes, but very few combinations are known or understood.

Finding the origins of disease in "this enormous sea of three billion letters" is a Herculean task, made harder by the fact that genes interact with other genes in complicated ways to create disease, he said. They can lie dormant for a lifetime, or be switched on by exposure to environment or lifestyle. Nevertheless, the genes that trigger inherited diseases are there, waiting to be found.

"We have to come up with a better way, or this isn't going to be feasible," he said. "Enter the haplotype idea. Basically, you can think of this as a short-cut."

Until recently, scientists had focussed on finding mutations in genes that caused rare diseases, and some 15,000 have been discovered. But for the more intractable ailments - those filling hospital wards - it's clear that slight variations in 'normal genes', rather than mutations, are more likely to be the trigger, he said.

And there are many, many small, seemingly innocuous variations in the human genome. Even though humans are 99.9% identical to each other, the last 0.1% still contains "millions of differences" to search through, Collins said.

But because some genes operate differently in some populations, and some populations appear to have a higher frequency of some diseases, scientists can use these clusters of differences - or variations - to pinpoint areas of the human genome likely to be key triggers for disease.

"We need to define the structure of human variation: a human haplotype map," Collins said. "There are genetic variants that contribute to common diseases. We need to find where in the genome are there areas that you can find at higher frequencies in people with disease."

Once these sites on the genome are found, drugs can be developed to target them, to treat the disease or even cure it, he said. Diseases where genes play a key role - and could be unmasked by the 'HapMap' project - are cancer, heart disease, diabetes and mental illnesses.

"With this resource, it's likely that we should be able to uncover the major contributing genes to the major illnesses, which we know have genetic origins, but which have remained out of our grasp," Collins said.

In order to map these variations, the International HapMap Project was launched in October last year. Researchers from the United States, Britain, Canada, China and Japan are participating. Thanks to improved technologies and the falling cost of genetic mapping, the project is expected to be completed by 2005 and cost around US$1 million.

All data collected will be available freely to anyone. But to prevent private companies from 'locking out' other researchers by using the data to patent genetic finds, all users have to first agree they will not do so - avoiding discoveries being claimed "by a third party in a parasitic fashion".

Collins said the 'Genome Era' was only just beginning, and the discoveries ahead give human beings significant control over diseases they had once thought untreatable. Most pharmaceuticals operate on about 500 known targets, whereas unravelling the human genome has theoretically made all 30,000 human genes a potential target.